Method for producing coprecipitated multicomponent oxide powder precursors

ABSTRACT

Guanidine oxalate is used as a precipitating agent in the coprecipitation of multicomponent oxide powder precursors. A solution of a soluble inorganic salt is combined with a solution of guanidine oxalate. A resultant metal oxalate is precipitated from the solution and can thereafter be subjected to heat to convert the metal oxalate into a multicomponent ceramic powder.

This is a continuation of U.S. application Ser. No. 07/620,019 filedNov. 30, 1990 now U.S. Pat. No. 5,252,314.

BACKGROUND OF THE INVENTION

This invention pertains to the art of ceramic powder precursors, andmore particularly to a method for producing coprecipitatedmulticomponent oxide powder precursors.

The invention is particularly applicable to a method for coprecipitatingmetal oxalates as precursors for multicomponent oxide powders used inproducing ceramics for a variety of applications, acid will be describedwith particular reference thereto. It will be appreciated, however, thatthe invention may be advantageously employed in other environments andapplications.

The coprecipitation of mixed salts from liquid solutions is awell-established method of ceramic powder precursor synthesis.Coprecipitation refers to the simultaneous precipitation of more thanone metal from the same solution.

A multicomponent liquid solution of soluble inorganic salts (e.g., metalnitrates, halides, sulfates) is typically combined with a liquidsolution of a precipitating agent compound. The precipitating agent ischosen such that, when dissolved and combined with the metals solution,one of its radicals combines with the metal ions to form insoluble saltswhich thermally decompose to form oxides. The insoluble salts willprecipitate in a very finely divided and intimately mixed state. Heatingthe precipitate decomposes these salts, resulting in a chemicallyhomogeneous, fine oxide powder with high surface area. This powder maythen be fabricated into a number of ceramic products using variousceramic fabrication techniques. Examples of such ceramic productsInclude, but are not limited to, electrical or electronic ceramics(integrated circuit substrates, capacitors, piezoelectric transducers,ferroelectric devices, or optical or optoelectronic devices, solidelectrolytes, electronically conductive ceramic electrodes, and ceramicsuperconductors); Magnetic ceramics (magnetic storage media, video oraudio tape heads, transformer cores, memory devices or arrays); ceramicsused primarily for their strength, hardness and/or chemical stability(refractories; heat exchangers; abrasives; fibers for reinforcement;bulk materials and coatings for protection from heat, oxidation,corrosion, wear, stress, or other physical or chemical changes; catalystsubstrates); pigments; and catalysts.

The type of precipitate formed depends oil the precipitating agent used.The precipitating agent can be selected from among a variety ofcompounds including, as examples, hydroxides, carbonates, and oxalates.Although there are advantages and disadvantages to using each of thevarious types of precipitating agents, precipitated carbonates andhydroxides in many cases tend to be gelatinous thereby difficult torinse, separate, and filter.

As a class, oxalates are generally highly insoluble, and they formparticles that are readily filtered from the liquid and easy to handle.For example, oxalates of the following compounds exhibit low watersolubility: Al, Ba, Bi, Cd, ca, Ce(III), Cr(II), Co, Cu, the rareearths, Ga, Fe(II), Pb, Mg, Mn, Hg, Nf, Ag, Sr, Tl(I), Th, U, Y, and Zn.

Coprecipitated oxide powder precursors offer increased homogeneity aswell as increased reactivity over those precursors which are notcoprecipitated. While coprecipitation is not the only way to achievethese advantages, the increase in homogeneity and reactivity isespecially advantageous in multicomponent oxide systems where theattainment of solid-state equilibrium is often slow, or when reactionsmust be carried out below a melting temperature. These conditionsprevail in the cuprate superconductors, for example, and severalcoprecipitation routes for their synthesis have been reported.Coprecipitation has also been used to produce magnetic oxide materials.

In current coprecipitation procedures, many of the precipitated saltsexhibit slight solubilities in the supernatant liquid. This leads toincomplete precipitations.

To insure a complete precipitation and the precise cation stoichiometrydesired, the pH of the mixture is controlled. In light of this, thereare several disadvantages associated with the prior art methods ofcoprecipitation.

First, if alkali metal hydroxides (e.g., NaOH, KOH etc.) are used toadjust the pH of the precipitating solution, extensive washings of theprecipitates are necessary to remove alkali metal residues which remainas contaminants in the final mixed oxide product. Similar contaminationof the powder by alkali metals can result from using alkali metaloxalates or carbonates (e. g. , Na₂ C₂ O₄ or Na₂ CO₃) as precipitatingagents.

Second, if aqueous ammonium hydroxide is used to neutralize the pH, orif ammonium oxalate is used as a precipitating agent, water-solubleammonia complexes of certain ions (e.g., copper, nickel or silver) canform.

Third, if weak organic bases are used, it is difficult to achieve a highenough pH required to quantitatively precipitate many oxalates unlesslarge quantities of the weak base are used in some cases, because oftheir relatively high molecular weights, the large amount of weak baseneeded becomes impractical. As is the case with ammonia, many weakorganic bases form soluble amine complexes with some cations such asCu⁺⁺ and Ni⁺⁺. This tends to prevent quantitative precipitation of thesecations.

Fourth, of the moderately strong organic bases, the tertiary amines,only trimethylamine, the first in the series, is practical in terms of aconvenient equivalent weight. The equivalent weights of the tertiaryamines increase rapidly upon going up in the series to triethylamine andtripropylamine. Trimethylamine, however, offers a few disadvantages inthat it is expensive to obtain and, in addition, has three carbon atomsper molecule which may leave a carbon residue, especially on firing in anonoxidizing atmosphere.

Of the very strong bases, the quaternary ammonium hydroxides, aboutequal in strength to sodium hydroxide, only the first in the series,tetramethylammonium hydroxide, is practical. Tetramethylammoniumhydroxide, nonetheless, the drawbacks similar to those found in usingtrimethylamine.

Fifth, a pH adjustment, subsequent to the Initial precipitation, leadsto a second precipitation. The second precipitate would probably have adifferent composition than the first. This could lead to some degree ofsegregation of the metals within the precipitate, as well as a loss ofcompositional homogeneity. For homogeneity of a coprecipitate, a singleone-time precipitation is advised.

Sixth, the solutions can be chilled or mixed with alcohol to decreasethe solubility of the precipitates. The alcohol can also act as anantifreeze enabling precipitations below 0° C. However, severalprecipitating agents have lower solubility in cold water or areinsoluble in alcohol. Examples of these precipitating agents include theoxalates of sodium, potassium, and ammonium.

It would be desirable to develop a method for quantitativelycoprecipitating multicomponent, chemically homogeneous oxide powderprecursors.

It would be further desirable to develop a method for coprecipitatingmulticomponent oxide powder precursors such that upon decomposition,intimately mixed, finely divided oxides with high surface area andchemical homogeneity would be produced.

The present invention demonstrates a new and improved method whichaddresses the above-referenced problems and others, and provides amethod for coprecipitating multicomponent ceramic powder precursors thatis simple, quantitative, and economical.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided a method forthe quantitative production of coprecipitated precursors formulticomponent oxide powders.

In accordance with a more limited aspect of the invention, a method forquantitatively coprecipitating metal oxalate precursors ofmulticomponent oxide powders is provided. That is, a solution of solubleinorganic salts is combined with a solution of guanidine oxalate. Metaloxalates forms and quantitatively precipitates from the resultantsolution. The precipitated metal oxalates are suited for subsequentconversion to a chemically homogeneous multicomponent metal oxide.

A principle advantage of the invention is that it provides aninexpensive, relatively simple method for quantitatively precipitatingchemically homogeneous metal oxalate precursors of multicomponent oxidepowders for ceramics.

Another advantage of the present invention is that the metal oxalateswhich are produced in accordance with the disclosed method aresubstantially free of contaminants which can leave residues in the finalmixed oxide product.

Still other advantages and benefits of the invention will becomeapparent to those skilled in the art upon a reading and understanding ofthe following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to the use of guanidine oxalate, alsoknown as guanidinium oxalate, [HN═C(NH₂)₂ ]₂ H₂ C₂ O₄, as aprecipitating agent for the preparation of oxalates which are precursorsto mixed oxide powders.

Guanidine oxalate is the oxalate of the strong base guanidine,NH═C(NH₂)₂. It can be synthesized inexpensively as it is easily made byadding a concentrated oxalic acid solution to a concentrated solution ofguanidine carbonate.

Advantages of using guanidine oxalate as a reagent for coprecipitatingmixed oxalates, precursors to metal oxides, over state of the arttechniques may be better understood by comparing its properties to thoseof commercially available water soluble oxalates. These commerciallyavailable oxalates include sodium oxalate, Na₂ C₂ O₄ ; potassiumoxalate, K₂ C₂ O₄ ; ammonium oxalate, (NH₄)₂ C₂ O₄ ; and oxalic acid, H₂C₂ O₄.

Among the major disadvantages of using Na₂ C₂ O₄ and K₂ C₂ O₄ asprecipitating agents is that these oxalates contain a non-volatilecation that is not readily removed by firing. Finely divided, freshlyprecipitated metal oxalates adsorb and occlude Na⁺ and K⁺ which canremain in significant amounts even after washing the precipitate. Na⁺and K⁺ will not be removed completely by firing; therefore, Na and Kcompounds remain as impurities in the final product. Such impurities aredetrimental to desirable high temperature and electronic properties ofmixed oxides.

While (NH₄)₂ C₂ O₄ contains the NH₄ ⁺ cation which is readily removed byfiring to provide a pure fired product free from contamination, (NH₄)₂C₂ O₄ has a number of disadvantages. First, (NH₄)₂ C₂ O₄ forms solubleammonia complexes with certain metal cations such as Ni, Co, and Cu.Thus, quantitative precipitation of oxalates involving these metals doesnot take place. Second, (NH₄)₂ C₂ O₄ has low solubility in both coldwater and cold water-alcohol systems. As a result, (NH₄)₂ C₂ O₄ cannotbe effectively used in cold aqueous systems. Some oxalates, such as SrC₂O₄, have decreased solubility in cold aqueous systems and are,therefore, more quantitatively precipitated at lower temperatures.

As with (NH₄)₂ C₂ O₄, oxalic acid fires to volatile products and leavesno residue contaminant in the fired material. Oxalic acid, however, hasa number of disadvantages. First, aqueous H₂ C₂ O₄ solutions are acidic.Generally, metal oxalates are significantly more soluble in acidsolutions than they are in slightly alkaline solutions. With H₂ C₂ O₄,quantitative oxalate precipitation cannot be accomplished in many cases.

Secondly, to reduce the solubility of metal oxalates, H₂ C₂ O₄ solutionsin ethanol have been used as precipitating reagents. However, because ofthe acidic nature of H₂ C₂ O₄, it is still necessary to raise the pH viaa subsequent addition of a basic solution. This second addition resultsin a second precipitation. The two-step process does not facilitate anideal coprecipitation. An effective coprecipitation should beaccomplished by a single operation where all of the metal salts andsolutions are quantitatively and simultaneously precipitated asoxalates.

Finally, oxalates of the strongly basic quaternary ammonium hydroxidessuch as tetramethylammonium oxalate, [(CH₃ ]₄ N]₂ C₂ O₄ (TMAOx) havebeen considered. TMAOx is the simplest of the quaternary oxalates interms of number of carbon atoms. The next in t)-ie series of quaternaryammonium oxalates, tetraethylammoniumoxalate, has too high a molecularweight to be practical as a reagent for oxalate precipitation.

TMAOx has most of the desirable properties of a good oxalateprecipitating reagent including high pH, non-complexing chemistry, goodsolubility in cold water, and it contains a volatile cation. TMAOX,however, has two major disadvantages when compared to guanidine oxalate.

First, the tetramethyl ammonium cation contains four carbon atomsinstead of one as in the guanidinium cation. This greater number ofcarbon atoms could lead to Incomplete pyrolysis with a greater tendencyto produce free carbon, especially if the pyrolysis is carried out at areduced temperature. Second, based on the cost of commercially availabletetramethylammonium salts required to synthesize TMAOX, this oxalatewould be considerably more expensive on a weight basis than guanidineoxalate. Guanidine oxalate is conveniently prepared from inexpensivecommercially available guanidine carbonate.

The use of guanidine oxalate as a precipitating agent offers severaladvantages. For instance, guanidine oxalate permits highly quantitative,one-step precipitations. That Is, since guanidine oxalate Is the salt ofa very strong base (equivalent in basicity to sodium hydroxide) and aweak acid (oxalic acid, HO₂ CCO₂ H), a solution of guanidine oxalate hasa high pH. When such a solution is added to acidic metal salt solutions(such as halide, perchlorate, or nitrate solutions), the resultingsupernatant is neutral or only slightly acidic. As a result, the need toneutralize the pH in a separate subsequent step is reduced oreliminated, while a quantitative oxalate precipitation is achieved.

Next, guanidine oxalate does not promote inorganic contamination.Because guanidine is organic, any guanidine salts (e.g., nitrates,perchlorates, chlorates) that may be carried down with the insolubleoxalates are completely pyrolyzed during the conversion by heat of theoxalates to oxides. (Guanidine chlorate, GHClO₃, is especially thermallyunstable).

It must be noted that salts of organic bases with oxidizing anions,nitrates, perchlorates and chlorates can undergo explosive thermaldecomposition; the nitrates are the least hazardous of the three in thisregard. Adequate precaution should be taken against violentdecomposition of absorbed perchlorates and chlorates during firing ofsuch oxalate precipitates. For a particular system involvingperchlorates or chlorates, it is advisable to test pyrolize smallsamples to determine the extent, if any, of an exothermic perchlorate orchlorate decomposition. Supernatent solutions containing perchlorates orchlorates should not be heated to dryness.

Guanidine oxalate Is a preferred precipitating agent because it is arelatively simple organic molecule that completely pyrolyzes in air at700° C. Thus, uncontaminated oxides are obtained, without washing orrinsing the precipitate.

Additionally, guanidine has a lower molecular weight (59.07 g/mol), andfewer carbon atoms (it has only one) than other strong organic bases.Therefore, guanidine oxalate yields a higher concentration of oxalateion per gram, and there is less organic material to be eliminated. Thisreduces the likelihood of carbon contamination in the oxide powder.

Also, use of guanidine oxalate as a precipitating agent promotes highyields. It permits one-step precipitation so that pH adjustment isunnecessary. Guanidine oxalate is very soluble both in cold water and inethanol-water solutions. Therefore, the volume of the supernatant can bereduced, and guanidine oxalate can be used advantageously for thoseoxalates which precipitate more completely in alcohol-water systems thanin purely aqueous systems.

Further, guanidine oxalate does not form soluble complexes with nickelor copper.

Aside from using guanidine oxalate as a precipitating agent, guanidinehydroxide itself may also be used as a strong base to replace NaOH, KOH,NH₄ OH, or basic organic amines. Guanidine carbonate may also be usedinstead of Na₂ CO₃ or K₂ CO₃ for the precipitation of insoluble metalcarbonates. Other guanidine salts may replace the corresponding sodiumor potassium salts where a volatile or combustible cation (e.g., theguanidine ion) is needed.

Hydrated guanidine oxalate has been synthesized and characterized usingdifferential thermal analysis (DTA), thermogravimetry (TGA), and x-raydiffraction (XRD). TI)e use of guanidine oxalate in the coprecipitationof several mixed oxide systems has been demonstrated. These systemsinclude manganese-zinc ferrites, yttrium-cerium oxides, and cupratesuperconducting compounds in the La-Sr-Cu-O, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-Osystems. The compositions of the supernatants and the precipitates weredetermined using proton-induced x-ray emission (PIXE), atomic absorption(AA), and inductively coupled plasma spectrophotometry (ICP). In somecases the precipitates were calcined and sintered, and characterized toconfirm their homogeneity, high reactivity, and composition.

Guanidine oxalate was synthesized by adding a stoichiometric amount ofconcentrated oxalic acid solution to a concentrated solution ofcommercially available guanidine carbonate (96%, Alfa Products):

    G.sub.2 H.sub.2 CO.sub.3 +H.sub.2 C.sub.2 O.sub.4 →guanidine oxalate+H.sub.2 O+CO.sub.2 ↑

The oxalic acid was added slowly to the carbonate so that the CO₂effervescence did not cause extensive foaming and loss of liquid.Afterward, the solution was evaporated to dryness below 1000° C. A whiteresidue was dissolved in a minimal amount of hot water, and stirred withactivated charcoal to remove any insoluble colloidal impurities from theguanidine carbonate. The clear solution was filtered, then evaporated todryness below 100° C., and stored.

Next, the guanidine oxalate was purified by recrystallizing it from anethanol/water solution. This was accomplished by first suspending thecrude guanidine oxalate in hot absolute ethanol, then adding water justuntil the guanidine oxalate dissolved. The hot, nearly saturatedsolution was cooled overnight to -100° C. to crystallize the oxalate.The crystals were filtered, washed with a small quantity of absoluteethanol, and dried at room temperature.

The powder x-ray diffraction (XRD) pattern of the crude guanidineoxalate did not match the published pattern for guanidine oxalatemonohydrate. After recrystallization, XRD showed that the material waspredominantly, but not entirely, the monohydrate. Weight measurementsindicated that there was more than one mole of water (either hydrated oroccluded) per mole of oxalate. Therefore, to avoid underestimating theamount of oxalate ion available during precipitations, the synthesizedmaterial was assumed to be a dehydrate.

Thermogravimetric analysis (TGA) of guanidine oxalate in air at aheating rate of 5° C./min showed that it was completely pyrolyzed (100%weight loss) at 700° C. TGA in helium at 5° C./min gave 99% weight lossat 800° C. The remainder was probably unreacted carbon, which would beburned off in air.

To demonstrate that guanidine oxalate does not form soluble complexeswith copper ions, the following test was performed. When the requiredamount of ammonium oxalate was added to a copper (II) nitrate solution,the supernatant above the precipitated copper oxalate was blue,Indicating the presence of dissolved copper-ammonia complex. The use ofguanidine oxalate instead of the ammonium salt yielded a clearsupernatant with a cu content of 6.6 ppm by weight.

Once the guanidine oxalate is synthesized, it is mixed with a solutionof cold water or cold water and alcohol. The temperature of the coldwater can range from about its freezing point to about room temperature(roughly 25° C.) or higher. Preferably, the temperature range of thewater is low, between about 0° C. and about 5° C. Similarly, thetemperature of the alcohol/water solution can range from about orslightly above its freezing point (below 0° C.) and room temperature(roughly 25° C.) or higher. The preferred temperature of use for thealcohol/water solution is about 50° C. and lower. These low temperaturesare preferred because the metal oxalates are less soluble at lowertemperatures.

The alcohol used in the water alcohol solution can be selected fromthose soluble in water and having a low molecular weight. Particularly,the alcohol can be selected from among methanol, ethanol, 1-propanol and2-propanol.

A solution of water and at least two types of soluble Inorganic salts isprepared and mixed with the above guanidine oxalate solution. Thesoluble inorganic salts can be selected from among sulfates, acetates,halides, nitrates, chlorates or perchlorates of Metals including AI, Ba,Bi, Cd, Ca, Ce(III), Cr(II), CO, Cu, the rare earths, Ga, Fe(II), Pb,Mg, Mn, Hg, Ni, Ag, Sr, TI(I), Th, U, Y, and Zn.

Once the two solutions are prepared, they are combined and stirred. Thetemperature of tile combined solution is within the ranges discussedabove with respect to the guanidine oxalate and water or alcohol/watersolutions. The guanidine oxalate and metal salts react to quantitativelycoprecipitate an intimate mixture of metal oxalates. A precipitate formsand falls to the bottom of the beaker which Is then centrifuged to morequickly separate the precipitate. The precipitate is removed, subjectedto filtration and dried in an oven to remove any excess water.Thereafter, the mixture of coprecipitated metal oxalates is fired in afurnace to remove any impurities, organic or otherwise, and to convertthe metal oxalates to an intimate mixture of metal oxides.

The invention will now be more particularly described by the followingExamples:

EXAMPLE 1

A first application of guanidine oxalate was in the precipitation ofprecursors to superconducting Bi-Sr-Ca-Cu oxides. The use of guanidineoxalate as a precipitating agent was compared to two other precipitatingagents: (1) tetrapropyl ammonium oxalate (TPAO), and (2) a nonaqueousethanol/oxalic acid solution (EtOH/H₂ C₂ O₄), with subsequent pHadjustment using ammonia dissolved in ethanol.

In the TPAO precipitation, Bi₂ O₃, SrCO₃, CaCO₃, and CuO were weighedout in amounts that would yield 20 g of Bi₂ Sr₂ CaCu₂ O₈. These weredissolved in 38 g of HNO₃ and 45 g H₂ O, and stirred until the solutionwas clear. Oxalic acid was added to a 10% aqueous solution oftetrapropyl ammonium hydroxide until the pH reached 8.8. Approximately30 ml of the metals solution was then added to roughly 300 ml of theoxalate solution while stirring. The mixture, which was at a temperatureof 23° C., was then centrifuged. The liquid was decanted and analyzed(see Table I) and its pH was 1.7. The solid was dried at 90° C.overnight and analyzed (see Table I).

For the guanidine oxalate and EtOH/H₂ C₂ O₄ precipitations, aBi:Sr:Ca:Cu solution with stoichiometry 2:2:2:3 was prepared by mixingBi(NO₃)₃ ·5H₂ O, Sr(NO₃)₂, Ca(NO₃)₂ ·4H₂ O, and Cu(NO₃)₂ ·3H₂ O, inamounts to yield 100 g of oxide, in 300 ml of deionized water. Anaqueous solution of 60 wt % HNO₃ was added gradually while stirringuntil the solution was clear, yielding approximately 435 ml of solution.

For the guanidine oxalate precipitation, 40 ml of the metals solutionwas added to a stirred solution of 40 g of guanidine oxalate in 1 literof water. The large excess (65%) of oxalate and large solution volumewas chosen to achieve a high pH (6.0 before the metals addition, 3.9after precipitation), and a fine particle size. The mixture wascentrifuged and the supernatant was analyzed.

For the EtOH/H₂ C₂ O₄ precipitation, the method followed was analogousto that reported in Sawano et. al., "Processing of SuperconductingCeramics for Flight Critical Current Density," Research Update1988--Ceramic Superconductors II, ed. M. F. Yan (1988), pg. 282-293.That is, 20 ml of the above solution was added to 500 ml of a solutionof 10% excess oxalic acid in ethanol at 5° C. The pH was 0.9 after theprecipitation began. A solution of ammonia in chilled ethanol was thenadded to raise the pH to 8.5; small additions of oxalic acid in ethanolwere then added to achieve a pH of 7.0. This precipitate settledextremely slowly. The mixture was centrifuged, and both the supernatantand the solid were analyzed. Results are set forth in Table I. Thesubscripts which appear under the Chemical Analysis sections of thetables below denote the uncertainty in the most significant digit.

                  TABLE I                                                         ______________________________________                                        System           Chemical Analysis                                            Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x                                             Method   Ca      Cu    Sr    Bi                                   ______________________________________                                        supernatant -                                                                             ICP      1       6.6.sub.2                                                                           .sup. 31.6.sub.1                           guan. oxal. (ppm)                                                             supernatant -                                                                             AA       640.sub.50                                                                            30.8.sub.3                                                                          390.sub.10                                                                          9.9.sub.1                            TPAO (ppm)                                                                    solid, cation %                                                                           (target) 14.3    28.6  28.6  28.6                                 TPAO        AA       18.2    29.1  23.4  29.3                                 supernatant -                                                                             AA        0.2    <1    <0.1  <1                                   H.sub.2 C.sub.2 O.sub.4 /EtOH                                                 (ppm)                                                                         solid, cation %                                                                           (target) 22.2    33.3  22.2  22.2                                 H.sub.2 C.sub.2 O.sub.4 /EtOH                                                             AA       20.9    35.6  20.9  22.7                                 ______________________________________                                    

Favorable results in the Bi-Sr-Ca-Cu-O system led to the exploration ofthe use of guanidine oxalate to coprecipitate several other mixed oxideprecursors of current scientific or technological interest. Thefollowing procedures were followed in all cases, unless otherwise noted,and all chemicals used were of reagent grade. One liter of metalssolution was added to 1.5 liters of cold guanidine oxalate solution,while stirring. The latter solution contained 10% excess oxalate ion.Stirring was continued for at least one hour. The insoluble oxalateswere recovered by centrifuging, then decanting the supernatant. Next,the supernatant was chemically analyzed for residual metal cations. Thesettled oxalate slurry was filtered by suction to remove excess liquid.

EXAMPLE 2

The recently discovered cuprate-based oxide superconductors particularlybenefit from two of the advantages of using guanidine oxalate: (1) theabsence of alkali contamination, which would be deleterious toelectrical properties; and (2) the avoidance of the formation of solubleCu complexes.

A liter of solution containing 0.15 mole Cu(NO₃)₂, 0.10 mole Ba (NO₃)₂,and 0.05 mole Y (NO₃)₂ (corresponding to the stoichiometry of YBa₂ Cu₃O_(7-x)) was prepared. This was added to the guanidine oxalate solutionat room temperature while stirring. The mixture was centrifuged, and thesupernatant was analyzed to determine the remainder of Cu, Y and Baremaining in the solution. The results are shown in Table II.

The experiment was repeated using similar cation ratios prepared fromacetate salts. Table II sets forth the results of this experiment.

                  TABLE II                                                        ______________________________________                                        System            Chemical Analysis                                           YBa.sub.2 Cu.sub.3 O.sub.7-x                                                               Method   Cu       Y     Ba                                       ______________________________________                                        supernatant -                                                                              ICP      1.8.sub.1                                                                              0.4.sub.1                                                                            5.4.sub.1                               nitrates (ppm)                                                                             PIXE     1.5.sub.1                                                                              <0.8  8.sub.2                                  supernatant -                                                                              ICP      3.4.sub.1                                                                              0.4.sub.1                                                                           30.06                                    acetates (ppm)                                                                             PIXE     3.2.sub.2                                                                              <0.8  14.sub.2                                 ______________________________________                                    

As can be seen, only a trace amount of Cu, Y, and Ba remained insolution. The precipitation of the metal oxalates was fairly complete.

EXAMPLE 3

Because the solubility of strontium oxalate increases significantly withincreasing temperature, the coprecipitation of the La-Sr-Cu system wasperformed in a cold water-alcohol solution. A cold, 1-liter solutioncontaining 0.05 mole each of LA(NO₃), La(NO₃)₃, and Cu(NO₃)₂ was addedto a guanidine oxalate solution containing 30% ethanol at below 0° C.Table III shows the results of the supernatant analysis.

                  TABLE III                                                       ______________________________________                                        System           Chemical Analysis                                            (La,Sr).sub.2 CuO.sub.4                                                                   Method   Cu        Sr   La                                        ______________________________________                                        supernatant ICP      1.0.sub.1 0.8.sub.1                                                                          0.2.sub.1                                 (ppm)                                                                         ______________________________________                                    

As in the case of the YB_(a) Cu₃ O_(7x) system of Example 2, only traceamounts of Cu, Sr and La remained in the supernatant. Metal oxalateprecipitation was substantially complete.

EXAMPLE 4

Because of the increasing solubility of cerium(III) oxalate withincreasing temperature, the precipitation of Y₂ Ce₈ O₁₉ was performed ina 30% ethanol-water solution of guanidine oxalate at 0° C. The starting1-liter solution contained 0.1 mole Ce(NO₃)₃ and 0.025 mole Y(NO₃)₃.Results of the supernatant analysis are shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        System             Chemical Analysis                                          Y.sub.2 Ce.sub.8 O.sub.19                                                                 Method     Y        Ce                                            ______________________________________                                        supernatant ICP        0.2      1.0                                                       PIXE       <1.2     <4.4                                          ______________________________________                                    

EXAMPLE 5

Mn₀.5 Zn₀.5 Fe₂ O₄ is typical composition of a commercially importantmagnetic oxide. It Is of the type used in magnetic recording heads,inductors, transformers, and video display tubes. The high solubility ofiron (III) oxalate in water makes it important to avoid forming theFE(III) ion.

Solutions containing 0.1 mole FeSO4, 0.025 mole MnSO₄ and 0.025 moleZnCl₂ were prepared. In a first precipitation, the solutions were keptcold, and a small amount of zinc dust was added to the solution toreduce any FE(III) to Fe(II). The added zinc apparently succeeded ateliminating Fe(III), as evidenced by the low Fe content In thesupernatant. The comparatively high Zn content may Indicate that some ofthe added Zn metal remained in the supernatant (see Table V).

                  TABLE V                                                         ______________________________________                                        System             Chemical Analysis                                          Mn.sub.0.5 Zn.sub.0.5 Fe.sub.2 O.sub.4                                                     Method    Mn       Fe     Zn                                     ______________________________________                                        supernatant -                                                                              ICP        3.7      2.2   71                                     cold water/Zn                                                                 supernatant -                                                                              ICP       188      263     8                                     using cold EtOH                                                                            PIXE       5.1.sub.4                                                                             126.sub.8                                                                             4.5.sub.3                             powder, cation %                                                                           (target)   16.67    66.67 16.67                                               PIXE       16.sub.1                                                                               65.sub.4                                                                            18.sub.1                               ______________________________________                                    

In a second precipitation, a cold ethanol/water solution of guanidineoxalate was used, without Zn dust. Results shown in Table V indicatethat this was less effective at aiding the precipitation of the Fe, ascomparatively large amounts of Fe remained in solution. Discussion

Tables I-V show the results of chemical analyses of the supernatants andprecipitates. As the results indicate, guanidine oxalate generallyachieved very complete precipitations, particularly of Y and the rareearths (La, Ce). The La-Sr-Cu precipitation, for example, achievedgreater than 99.9% precipitation of the metals (from an initial metalsconcentration of 0.6% in the total system, to 2 ppm total metals in thesupernatant), as did the Y-Ce precipitation. Where two analyses of thesame liquid were performed, there was substantial agreement betweenanalytical techniques.

In the comparison to other oxalate coprecipitations of BI-Sr-Ca-Cu-Oprecursors, Table I shows that the TPAO precipitation left severalhundred ppm of Ca and Sr in solution. The solid (unexpectedly) showed alarge excess of Ca, suggesting that it was not compositionallyhomogeneous. The EtOH/H₂ C₂ O₄ precipitation was highly quantitative(less than or equal to 1 ppm total metals), as would be expected fromthe extraordinary precautions involved: a nearly non-aqueous system,chilled, with ammonia pH adjustment. However, the precipitation occurredin multiple stages, with additional precipitate forming after each pHcorrection.

In comparison, the single-step aqueous guanidine oxalatecoprecipitation, at room temperature and without pH adjustment, gavemetals concentrations in the supernatant less than 10 ppm for allcomponents except Bi, which was at nearly 32 ppm (Table 1).

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon a reading and understanding of this specification. It isintended to include all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof.

We claim:
 1. A method for coprecipitating metal oxalate precursors formulticomponent metal oxide powders, comprising the steps of:synthesizingand isolating guanidine oxalate; purifying the isolated guanidineoxalate by recrystallization; dissolving purified guanidine oxalate inwater at a temperature below about 25° C. to provide a guanidine oxalatesolution; mixing the guanidine oxalate solution with a solutioncomprising a soluble metal salt and water to form a resulting solution;and precipitating a metal oxalate from the resulting solution.
 2. Amethod for coprecipitating metal oxalate precursors for multicomponentmetal oxide powders, according to claim 1, wherein the soluble metalsalt includes an oxidizing anion.
 3. A method for coprecipitating metaloxalate precursors for multicomponent metal oxide powders, according toclaim 1, wherein the guanidine oxalate solution is comprised ofguanidine oxalate, water and a water soluble alcohol having 1-3 carbonatoms per molecule.
 4. A method for coprecipitating metal oxalateprecursors for multicomponent metal oxide powders, according to claim 3,wherein the alcohol is ethanol.
 5. A method for coprecipitating metaloxalate precursors for multicomponent metal oxide powders, according toclaim 3, wherein the alcohol is methanol.
 6. A method forcoprecipitating metal oxalate precursors for multicomponent metal oxidepowders, according to claim 1, wherein the step of purifying includesthe steps of:suspending guanidine oxalate in hot alcohol; adding waterto dissolve the guanidine oxalate to form a nearly saturated solution;and cooling the nearly saturated solution to drystallize the guanidineoxalate.
 7. A method for precipitating multicomponent oxide precursors,comprising the steps of:synthesizing stoichiometric guanidine oxalateaccording to a process including a step of adding oxalic acid to asolution of guanidine carbonate; purifying the stoichiometric guanidineoxalate; adding the purified guanidine oxalate to a solution comprisingwater or water and alcohol to provide a guanidine oxalate solution;mixing the guanidine oxalate solution with an aqueous soluble metal saltsolution to form a resultant solution having a temperature of less thanabout 25° C.; precipitating a substantially homogeneous metal oxalateproduct from said resulting solution.
 8. A method for coprecipitatingmulticomponent oxide precursors, according to claim 7, wherein thesoluble inorganic salt solution includes more than one type of metalion.
 9. A method for coprecipitating multicomponent oxide precursors,according to claim 8, wherein more than one type of metal oxalateprecipitates from the resultant solution.
 10. A method forcoprecipitating multicomponent oxide precursors, according to claim 7,wherein the guanidine oxalate is synthesized according to the stepsof:adding a stoichiometric amount of concentrated oxalic acid solutionto a concentrated solution of guanidine carbonate to provide a resultantsolution; evaporating the resultant solution to leave a residue;dissolving the residue in water; removing insoluble colloidalimpurities; and filtering the guanidine oxalate from the solution.
 11. Amethod for precipitating multicomponent oxide precursors, according toclaim 7, wherein the step of purifying includes the stepof:recrystallizing the guanidine oxalate from an alcohol/water solution.